I've also written some Dossiers for a few of the suggested creatures:
Tame method: steal egg
Role: battle mount; versatile travel
Like its close relatives in the desert, Draconis oceanum is a formidable foe. Unlike its relatives, however, D. oceanum lives primarily in the deep oceans. It is similar in shape to its desert dwelling relatives, though its tail is much more eel-like and it has a pair of flippers rather than legs, making it an extraordinarily good swimmer. It is also capable of sustained flight in open air. The only downfall seems to be its near inability to walk on land, due to the lack of any real legs.
It is much darker in color than its cousins, appearing in deep shades of blue, purple, or black. It also has a row of bioluminescent spots running along each side of the dorsal fin and at the ends of four “whiskers” on its head, similar in fashion to the angler fish. The exact purpose of these is unclear as it appears to be a voracious and active hunter, rather than an ambush predator like the angler. Domesticated D. oceanum is as deadly as it is versatile. While in the water, it is able to emit a high-frequency sonic blast that stuns most other creatures, including survivors. While on land or in flight, it switches to attacking with a superheated stream of seawater which seeps into foes’ armor.
While highly prized among warrior tribes, a beast such as this is not easy to obtain. Like its relatives, it can only be tamed by stealing an egg from a nest, which just so happen to be located in extremely toxic underwater lakes.
Tame method: knockout (fish)
Behavior: aggressive; ambush predator
Role: battle mount; transporter
Wild Sepia gigantes, or Giant Cuttlefish, are masters of disguise. Though much larger than common cuttlefish, they still possess the unique ability to change appearance rapidly. This ability is often used for camouflage. It is also useful for confusing prey and predator alike, although domesticated animals are usually trained well enough to recognize the deception.
The Giant Cuttlefish is also capable of lashing out with its feeder tentacles, which are much longer than the others. It can then grab prey and pull them in to chew on them or simply hold onto them. Domesticated S. gigantes is highly prized for its surprising ability to extend its camouflage over its saddle and rider. This, combined with their ability to lash out and grab smaller creatures and survivors, makes them excellent guerilla mounts. S. gigantes is also incredibly mobile, able to move equally well forwards and backwards, even strafing side to side. Like its cousin, Tusotuethis, the Giant Cuttlefish can escape from bad situations with a quick jet of sticky black ink.
Tame method: passive (Hatz egg)
Behavior: neutral; loyal
Role: battle mount; versatile travel
Though much smaller, Draconis praeceps is no less threatening than its cave dwelling cousin. What it lacks in strength, it makes up for with speed and numbers. D. praeceps is a pack creature, commonly seen in groups of three or four individuals. It is amazingly fast at climbing and gliding among the steep cliffs where it nests and there is little hope of outrunning them on foot. Even the water is not safe, as these creatures are just as quick in the water and can seemingly hold their breath indefinitely.
Fortunately, D. praeceps is not typically aggressive unless disturbed. Or you happen to be a fish, of course. Domesticated
While there does not appear to be any alpha, these creatures are incredibly loyal to their pack. Often, when one is tamed, packmates will join the survivor as well. Larger packs are more likely to splinter into multiple packs, with those not joining the tamed individual becoming fiercely aggressive to those who did.
Survivors who are lucky enough to survive this attack and win the loyalty of a pack will have earned themselves a formidable army of versatile mounts, whether making war or simply exploring.
Although, to even think about taming these creatures, you’ll first have to procure a bounty of Hatzegopteryx eggs. This delicacy seems to be the only way to buy their loyalty.
Tame method: passive (meat)
Role: resource harvester
The “abnormal shrimp” is certainly a strange-looking creature with its rather long, flat, finned body and two large feeding arms. The feeding arms of Anomalocaris canadensis are exceptionally strong, being able to tear apart trilobites and clams which form the bulk of its diet.
While they aren’t immune to the toxicity of underwater brine lakes, Anomalocaris can often be found near them. They sometimes appear in shallow water as well, looking for a tasty trilobite snack.
Anomalocaris are not aggressive creatures unless disturbed. If one finds themselves incurring the wrath of one these creatures, it is best to flee. They can inflict an incredibly painful bite, but will not follow long distances. Domesticated
Anomalocaris are not large enough to ride, but that hasn’t stopped tribes from taming them. These creatures are not as brainless as they appear. Much like the land-dwelling Moschops, Anomalocaris can be taught to harvest specific resources. They have a particular affinity for gathering chitin and pearls, but can be taught to harvest stone or even metal!
Tame method: knockout (fish and/or biotoxin)
Role: passive healer; resource gatherer
Wild Archelon therapeftis is a truly amazing creature. It glides effortlessly through the water chasing down crustaceans and digging up clams, crushing them with its powerful jaws. I’ve even seen them munching on cnidarians, seemingly oblivious to their painful stings. In fact, Archelon appears to be able to process their toxins into an oily substance that seems to ooze from their skin, filling the water around them. This substance provides a rejuvenating effect that allows them to shrug off most adverse effects aside from direct physical damage. Interestingly, this secretion is able to heal other creatures as well. Domesticated
While Archelon’s shell is not nearly as tough as that of Carbonemys, it still provides a good bit of protection, making it a durable aquatic mount. However, the real reason tribes tame these is the rejuvenating effects of the secretions they produce. The secretions break down too fast to be able to collect and keep, but Archelon can keep producing them as long as it has a steady supply of biotoxin.
In addition to this healing ability, the Archelon can use its powerful beak to harvest chitin, stone, and even metal, making it an extremely useful mount. To make it even better, the extra weight doesn’t seem to affect its ability to swim, effectively reducing the weight of these materials.
Tame Method: knockout (algae/kelp)
Role: algae/kelp harvester; transport
Equally at home in the water as it is on land, Moeritherium lyonsi spends most of its time grazing in the shallow swamps. It is not a fussy eater and will happily munch on aquatic vegetation, berries, and even mushrooms. These beasts are generally passive toward most creatures, including survivors. However, when threatened, they can become very dangerous. They may not look it, but they are surprisingly fast on land. Luckily, they are too lazy to hold a grudge and will quickly forget any attacker that decides to run. Domesticated
Many tribes use Moeritherium as beasts of burden, being able to carry a fairly significant amount of weight. They are decently fast over short distances and excellent swimmers, making them superb at all-terrain transport. Some tribes also farm them for meat.
Their sensitive snout also makes them very adept at gathering berries and algae.
Desmatosuchus Tame method: knockout (algae/kelp/vegetables) Behavior: passive Role: algae/kelp harvester; versatile travel; loot gatherer
In stark contrast to both Kaprosuchus and Sarcosuchus, Desmatosuchus spurensis is a very passive animal. It prefers digging in the mud for tasty vegetation over hunting down prey like its cousins. That doesn’t mean that it isn’t a threat, however. Desmatosuchus can still deliver a rather painful bite and its strong neck muscles, which were developed for digging up roots, can fling survivors and small creatures great distances. Domesticated Desmatosuchus is about the same size as a Kaprosuchus and therefore is often used as a more easily obtained mount. Some survivors even ride bareback, due to the shoulder spikes providing a good place to grip. Though it is much less dangerous to tame, Desmatosuchus is just as efficient at traveling both land and water. Some tribes employ Desmatosuchus to assist in gathering edible wild tubers and roots from the swamps which their flat snouts and strong neck excel at doing. Left on their own to wander around, they can sometimes even dig up valuable items lost long ago to the muddy swamp.
Pakasuchus Tame method: passive (meat) Behavior: fearless [aggressive (male); passive (female) Role: shoulder pet
Wild Pakasuchus atromitos is a rather small crocodilian, no bigger than a common housecat. Aside from size, it is nearly identical in appearance to Kaprosuchus. Much like housecats, they have a fondness for climbing trees. Who would ever think to look up to find crocodiles! Thankfully, they are not particularly dangerous, especially the females who are generally passive. The males have an unusual lack of fear given their size and will attack just about anything that enters their territory, but even they are not really dangerous due to their small size. Domesticated Pakasuchus seem to server little purpose and most survivors will just completely ignore them, only defending themselves against the males. They provide little meat and while their hides are tough, there’s not much of that either. They cannot be used as mounts and are not particularly strong, yet a few tribes have found some use in their affinity for climbing. They tame or breed vast numbers of Pakasuchus and train them to climb over wood or even stone fences and walls to attack and harass unsuspecting rivals. Other survivors seem to just want a Pakasuchus for companionship. This trend has earned it the name Swamp Cat among many such survivors. Some Pakasuchus even form strong enough bonds that they will bring gifts to their masters in the form of insects, small fish, or even the occasional dodo.
Tame method: knockout (fish)
Role: temperature regulation; fish gatherer
Very similar in appearance to large modern seals or walruses, Thalassoleon thermenktis lives in the snowy islands to the south. Its thick coat of fur is not only near impervious to the freezing temperatures, but also gives the beast substantial protection against most primitive ranged weapons such as a bow and arrow.
Thalassoleon is fairly slow on land, but in the water few creatures can match it. Its speed is impressive and it even seems to become stronger and more aggressive, especially while feeding on schools of fish. I don’t recommend approaching these creatures during a feeding frenzy. You will regret it. Even Megalodons don’t dare interrupt a group of feeding Thalassoleons.
On land, they are rarely aggressive, only becoming hostile if one gets too close. Domesticated
Their exceptional swimming ability makes Thalassoleon a prized mount for any seafaring tribe. They can carry a fair amount of supplies and their ability to travel on land (albeit slowly) makes them decent transport mounts. Some tribes use them for fishing as well.
Another great benefit of these creatures is their fantastic ability to insulate themselves and their riders against temperature extremes. Their thick layer of fur provides exceptional protection against the cold and even insulates against heat.
Tame method: knockout (meat)
Role: battle mount; transport
Larger than a Pteranodon and smaller than a Quetzlcoatlus, Hatzegopteryx ourantromos is much more dangerous than either. It is fast, powerful, and very aggressive. Hatzegopteryx normally preys on fish, often diving and swimming for short distances. However, it will not hesitate to attack and consume smaller land creatures that wander into its territory. When Hatzegopteryx locks onto its prey, it will dive like a falcon often slamming its prey into the ground, pinning them and tearing them to pieces with its sharp beak.
Hatzegopteryx tend to stick to the mountains where they live. They tend to spend most of their time guarding their nests from the ever-present threat of the Sea Drakes they share their mountainous home with, but will sometimes travel great distances in search of prey. Domesticated
Because of their strength and speed in comparison to other flying mounts, many tribes tame Hatzegopteryx as an aerial battle mount, often times utilizing a three-seater saddle. This makes for a terrifying adversary in battle with the two “gunner” seats defending the naturally deadly creature.
This same three-seater design on the saddle, makes Hatzegopteryx an ideal beast of burden. It can carry a substantial amount of goods while still being defensible from would-be attackers.
Taming method: knockout (meat)
Role: early battle mount; travel; prime fish source
Wild Xiphactinus almatromos patrols shallow waters near reefs and drop offs, even traveling up into river channels. It is a rather large fish, though not nearly as large as a Megalodon, and highly aggressive, preying on fish or birds and other animals that happen to venture out into the water. Its aggressive nature means it is a solitary creature, even taking to cannibalism when encountering another of its own kind.
They are also very fast and agile, often leaping from the water to attack low flying birds. I have even heard reports of Xiphactinus leaping from the water and snatching people right off their rafts! Domesticated
Easily large enough to ride, many survivors tame these as their first aquatic mount. They are reasonably well-equipped to defend their riders and incredibly fast. Others use them not as mounts, but as escorts. When tamed, they can be trained well enough that they do not resort to cannibalism in the presence of their own kind as they do in the wild, although when too many are kept together their instincts will become too strong and they will slaughter each other.
Many tribes also hunt them for their meat. I can personally attest to their marvelous flavor when fried with a little bit of citronal juice…
Taming method: knockout (meat)
Role: ambush defense (like purlovia)
Wild Gerrothorax diavammos is a particularly dangerous creature, not so much because they are large or powerful, but because they are almost impossible to see before it’s too late. They hide themselves in the sandy ocean bottom waiting for unsuspecting prey to swim past, be it a fish or an unfortunate survivor. They typically prefer smaller fish that can be swallowed in a single gulp, but a hungry Gerrothorax will have no problem bashing larger prey items with its thick, and very sharp, wedge-shaped skull. It lacks teeth, but is capable of delivering a very powerful bite.
Gerrothorax is very patient and doesn’t like to wander. They will often pick a spot and remain there for exceptionally long periods of time, bursting from the sand to gulp prey and immediately returning to their hiding spot to digest. They will only wander if prey becomes scarce or they are looking for a mate. Domesticated
Just barely large enough to ride, Gerrothorax are often capable of digging deep enough to conceal their riders. This, combined with their ability to lunge at surprising speeds, makes them useful in setting ambushes. However, their relative fragility and slow swimming speed renders them impractical for much else.
I have seen some tribes employ Gerrothorax as a sort of perimeter defense around their underwater settlements (the likes of which still amaze me) where their slow metabolism allows them to remain hidden for weeks or even months, according to some, without being fed.
Taming method: knockout (algae)
Time: Early Cretaceous
Roles: transport; battle-mount
Wild Amargasaurus anapnerou is one of the smaller sauropods I’ve seen. And one of the most unusual. It has a double row of spines that runs down its neck and continues to a lesser degree down its back all the way to the tail. The spines on the neck are much larger and sport a double sail, which seems to be mostly for show. The spines themselves, however, make for very dangerous weapons.
The spines along the back conceal and protect a large air sack, which gives Amargasaurus the ability to graze on underwater vegetation for very long periods without surfacing for air.
Amargasaurus is an excellent swimmer for a sauropod. They are capable of swimming short distances easily, but if the distance is too great to swim all at once, they simply sink to the bottom and rest or walk the remaining distance. Provided the water isn’t too deep, of course. You won’t see them swimming or walking the open ocean. Domesticated
While smaller than the mighty Bronto or even the Diplodocus, Amargasaurus still makes an excellent transport mount due to its greater affinity for water than either of those two. It is capable of carrying a great amount of goods and some tribes have even crafted large storage compartments into their saddles to increase the amount of goods they can transport.
Other tribes see Amargasaurus as an amphibious war tank, utilizing its strength and mobility to carry multiple passengers, often outfitting it with a heavily armored five-seater saddle.